An integrated circuit requires conductive interconnects between semiconducting domains in order to communicate signals therebetween. In order to create ever faster microprocessors, smaller dimension interconnects of higher conductivity materials is an ongoing goal.
As microelectronic efficiencies have increased interconnects have decreased in dimensional size and efforts have been made to increase the electrical conductivity of interconnect features. There is an ongoing need for ever smaller interconnects.
The rapid miniaturization of interconnects is occurring simultaneously with the transition from Al to Cu metallization for sub-0.25 .mu.m ICs. The transition from Al to Cu has led to a change in the way interconnects are formed. While Al has been deposited as a blanket layer which is then patterned by reactive ion etching, Cu interconnects are formed by evaporative deposition into preformed (damascene) trenches and vias followed by chemical mechanical polishing (CMP).
As the interconnect width decreases and the aspect ratio increases, conventional vacuum deposition techniques approach the theoretical resolution threshold. Deep, narrow trenches and vias preferentially collect material at the damascene feature edges, leading to void formation. Blanket and selective chemical vapor deposition (CVD) are well established Cu deposition techniques that have a demonstrated ability to fill current interconnect trenches. (A. E. Kaloyeros and M. A. Fury, MRS Bull., (Jun. 1993), pp. 22-29).
Nonetheless, CVD does not inherently fill trenches preferentially over any other portion of substrate having nucleation sites. Unlike CVD, the proposed method preferentially deposits Cu into trenches based on differential solvent evaporation associated with trenches and as such is expected to work better, the narrower the trench width and higher the aspect ratio. Additionally, heating of the IC substrate during CVD to assure crystalline growth degrades fine architecture structures on the substrate. Thus, the semiconductor industry is in need of an interconnect formation process capable of achieving higher resolution at lower temperature and ideally, at a lower cost.
The mesoscopic size regime between atoms and bulk materials is characterized by unusual properties. Mesoscopic systems exhibit collective atomic behavior, but not to a sufficient extent so as to preclude quantized effects. Many of the unusual thermodynamic and spectroscopic anomalies associated with mesoscopic systems are attributable to surface effects. Studies have shown surface energies that are 10 to 400% greater for nanocrystals than for bulk Au and Pt, (C. Solliard and M. Flueli, Surf. Sci. 156, (1985), pp. 487-494) and Al (J. Wolterdorf, A. S. Nepijko and E. Pippel, Sur. Sci. 106, (1981), pp. 64-72). In the bulk, surface atoms represent such a small percentage of the total that surface effects are largely inconsequential. Surfaces generally possess modified atomic coordination numbers, geometries and diminished lattice energies relative to the bulk. The result of these modifications is that physical, spectroscopic, and thermodynamic properties, which are constant in the bulk, become size dependent variables in nanocrystals. The ability to modify the thermodynamic properties of nanocrystals, particularly the melting temperature, is exploited in the present invention to produce thin film IC structures at low temperature.
Metallic nanocrystals have been shown to reduce melting temperatures compared with the bulk. (Ph. Buffat and J-P. Borel, Phys. Rev. A, 13, (1976), pp. 2287-2298. 2287-2298; C. J. Coombes, J. Phys., 2, (1972), pp. 441-449; J. Eckert, J. C. Holzer, C. C. Ahn, Z. Fu and W. L. Johnson, Nanostruct. Matls., 2, (1993). 407-413.; C. R. M. Wronski, Brit. J. Appl. Phys. 18, (1967), pp. 1731-1737 and M. Wautelet, J. Phys. D, 24, (1991), 343-346). The depression in melting and annealing temperature is evident throughout the nanocrystal size regime, with the most dramatic effects observed in nanocrystals having a diameter from 2 to 6 nm. Melting studies on a range of nanocrystals have established that the melting temperature is size dependent in the nanometer size regime and is approximately proportional to the inverse particle radius regardless of the material identity. The size dependent melting temperature of metallic nanocrystals has included studies of Au, Pb and In, Al and Sn. (Au: Ph. Buffat and J-P. Borel, Phys. Rev. A, 13, (1976), 2287-2298. 2287-2298; Pb and In: C. J. Coombes, J. Phys., 2, (1972), 441-449; Al: J. Eckert, J. C. Holzer, C. C. Ahn Z. Fu and W. L. Johnson, Nanostruct. Matls, 2, (1993), 407-413; and Sn: C. R. M. Wronski, Brit. J. Appl. Phys. 18, (1967), 1731-1737). The reduction in melting temperature as a function of nanocrystal size can be enormous. For example, 2 nm Au nanocrystals melt at about 300.degree. C., as compared to 1065.degree. C. for bulk gold. (M. Wautelet, J. Phys. D, 24, (1991), 343-346).